Writing medium for electrostatic printing



y 8, 1968 E B. @QELPH 3,385,730

WRITING MEDIUM FOR ELECTROSTATIC PRINTING Filed April 1, 1964 10 \Z\\\\\\\\\\\I\\\\\\\\ Dielectric t ng Paper Base .Sheei carzkz'zrzz'rAmmonium l2 Chloride and @{ycerinc INVENTD/Z y 'arl B. Eel D 1 UnitedStates Patent 3,385,730 WRITING MEDIUM FOR ELECTROSTATIC PRINTING EarlB. Ralph, Des Plaines, 11]., assignor to A. B. Dick Company, Niles,Ill., a corporation of Illinois Filed Apr. 1, 1964, Ser. No. 356,382 5Claims. (Cl. 11720l) ABSTRACT OF THE DISCLOSURE The invention isaddressed to an electrostatic writing medium adapted to receive a latentelectrostatic image capable of subsequent development by a liquid orpowdered developer and in which .the electrostatic writing medium isformed with a dielectric insulating coating on the surface of a paperbase sheet which is impregnated for uniform distribution of thecombination of a polyhydric alcohol and ammonium choride.

This invention relates to a writing medium for use in electrostatic drycopy processes and it relates more particularly to a coated paper onwhich a latent electrostatic image can be formed for subsequentdevelopment by conventional dry or liquid toners.

Writing media for dry electrostatic printing processes are generallyformed of a dielectric coating on an electrically conductive basewhereby the latent electrostatic image is received on the surface of thedielectric coating for retention until developed by the dry or liquidtoner. It is preferred to fabricate such writing media of a dielectriccoating applied onto a paper base sheet but the combination is foundoften times to be deficient by reason of the variations occurring in theelectrical conductivity of the paper base sheet, depending somewhat uponthe composition of the paper, the relative humidity conditions existingat the time of copy preparation, the previous history of the coated copypaper, and many other factors.

It is an object of this invention to produce a writing medium which ischaracterized by low cost and ready availability; which can bemanufactured of a dielectric coating on a paper base sheet whichexhibits good electrical conductivity independent of the previoushistory of the writing medium in storage and substantially independentof the humidity conditions existing in the ambient atmosphere at thetime of copy production; which exhibits improved electricalconductivity, and which can be used to produce copy of good qualityconsistently, from batch to batch, with very little variation or withlittle sensitivity to the wide changes occurring in atmosphericconditions.

These and other objects and advantages of this invention willhereinafter appear and for purposes of illustration, but not oflimitation, an embodiment of this invention is shown in the accompanyingdrawing, in which:

FIG. 1 is a perspective view partially in section of a writing mediumembodying the features of this invention.

In order to minimize these variables and to produce a coated paperwriting medium having improved electrical conductivity in the base sheeton which the dielectric coating is formed, attempts have been made toformulate the base sheet of materials which would increase moistureretention and control independent of the humidity in the ambientatmosphere and which would impart higher electrical conductivity. In mypreviously issued Patent No. 3,075,859, description is made of anelectrostatic writing medium in the form of a dielectric coating on apaper base sheet wherein the latter is formulated to contain a humectantor ionizable metal salt, and preferably the combination of a humectantand an ionizable metal salt "ice uniformly distributed throughout thebase sheet to render the base sheet substantially insensitive to changesin humidity conditions in the ambient atmosphere and to improve theelectrical conductivity of the base sheet and thereby to produce animproved writing medium for dry electrostatic printing.

It has now been found, in accordance with the practice of thisinvention, that the specific combination of ammonium chloride and apolyhydric alcohol which, when uniformly distributed substantiallythroughout the base sheet, will provide optimum characteristics in thebase sheet for use as an electrostatic writing medium when provided witha suitable dielectric coating.

Best results are secured with the combination of glycerine as thepolyhydric alcohol and ammonium chloride present in the ratio of 1 partby weight ammonium chloride to 2 /2 parts by weight of glycerine but theratio can be varied within the range of 1 part by weight of ammoniumchloride to 1 to 4 parts by weight of glycerine. Instead of glycerine,use can be made of other polyhydric alcohols such as ethylene glycol,dior triethylene glycol, propylene glycol, pent-a erythritol, sorbitoland the like, in corresponding amounts.

It is desirable to incorporate an amount of the mixture of glycerine andammonium chloride into the paper base sheet suflicient to provide thedesired stabilization and improved electrical conductivity but not somuch as will squeeze out of the base sheet to wet .the adjacent surfaceof the dielectric coating, when the sheets are plied one on the other,as in a stack or when rolled, thereby to interfere with the desireddielectric properties of the coating and the ability of the coating toreceive and retain good definition in the latent electrostatic image.The amount of ammonium chloride and glycerine will depend somewhat uponthe basis weight of the paper, the amount of moisture left in the paper,and the conditions under which the sheet is packed or otherwise left inthe roll. In practice, with a coating composition formulated inaccordance with the following Example 1, it is undesirable to exceed acoating weight of 12 pounds per 3000 square feet of surface area but itis desirable to make use of an amount of at least 2 pounds per 3000square feet of surface area. Optimum results are achieved by theapplication of the materials in a coating weight of 3 to 7 poundsglycerine-ammonium chloride per 3000 square feet. When calculated onpercent by weight of the paper base sheet, the broad range is 0.1 to 35percent by weight of the polyhydric alcohol and preferably 0.1 to '15percent by weight of ammonium chloride and 0.1 to 20 percent by weightof the polyhydric alcohol.

The following is given by way of illustration, but not by way oflimitation, of the preparation of an electrostatic writing mediumembodying the features of this invention:

EXAMPLE 1 Treating composition: Percent by weight Glycerine 25 Ammoniumchloride 10 Water 62 Butanol 3 The above composition is applied to asuitable paper base sheet, preferably after coating with a dielectricmaterial, otherwise ammonium chloride is dissolved out of the paper intothe coating whereby it might be elfective to reduce the resistivity ofthe dielectric. If the salt is not soluble in the dielectric coatingcomposition or if a barrier coating is employed, between the dielectriccoating and the paper, then the treating composition can be appliedbefore coating. The composition is applied in coating weights of 3 to 7pounds of solids embodying may be formulated of such synthetic resinousmaterials as polyvinyl acetate, polyvinyl chloride, polyvinylidenechloride, polyethylene, polyacrylic acid esters, polyvinyl acetatecopolymers such as polyvinyl acetate-crotonic acid copolymer,polyesters, alkyd resins, oil modified alkyd resins, shellac,microcrystalline wax, petroleum hydrocarbon resins, :butadiene-styrenecopolymers, nitrocellulose, cellulose acetate, and epoxy resins.

Thus the electrostatic writing medium of this invention is illustratedin FIG. 1 wherein the numeral represents the dielectric coating on thesurface of a paper base sheet 12 containing a mixture of ammoniumchloride and glycerine uniformly distributed throughout the paper basesheet in the amounts described.

It will be apparent from the foregoing that I have provided a specificformulation of materials incorporated in a paper base sheet of anelectrostatic writing medium for improvement of the characteristics ofthe paper base sheet whereby the Writing medium becomes less sensitiveto variations normally occurring in the ambient atmosphere rand wherebythe paper base sheet is characterized by a desirable high electricalconductivity which it retains over an extended period of time.

It will be understood that changes may be made in the details offormulation and application and in composition, without departing from.the spirit of the invention, especially as defined in the followingclaims.

I claim:

1. An electrostatic Writing medium on which a latent electrostatic imageis formed and developed, comprising the combination of a paper basesheet and a continuous dielectric insulating coating on the paper basesheet, and a polyhydric alcohol and ammonium chloride substantiallyuniformly distributed throughout the paper base sheet in which thepolyhydric alcohol and ammonium chloride are present in the ratio of 1part by weight of ammonium chloride to 1 to 4 parts by weight of thepolyhydric alcohol and in which the materials are present in the basesheet in an amount within the range of 2 to 12 pounds by weight of themixture of polyhydric alcohol and ammonium chloride per 3000 square feetof surface area.

2. An electrostatic writing medium 'as claimed in claim 1 in which thepolyhydric alcohol is glycerine.

3. An electrostatic writing medium on which a latent electrostatic imageis formed and developed, comprising the combination of a paper basesheet and a continuous dielectric insulating coating on the paper basesheet, and :a polyhydric alcohol and ammonium chloride substantiallyuniformly distributed throughout the paper base sheet in which thepolyhydric alcohol and ammonium chloride are present in the ratio of 1part by Weight of ammonium chloride to 1 t0 4 parts by weight of thepolyhydric alcohol and in which the materials are present in the basesheet in :an amount within the range of 3 to 7 pounds by weight ofsolids composed of polyhydric ralcohol and ammonium chloride per 3000square feet of surface area..

4. An electrostatic writing medium on which a latent electrostatic imageis formed and developed, comprising the combination of a paper basesheet and a continuous dielectric insulating coating on the paper basesheet, and a polyhydric alcohol and ammonium chloride substantiallyuniformly distributed throughout the paper base sheet in which theammonium chloride is present in an amount within the range of 0.1 to 25percent 'by weight of the paper base sheet and in which the polyhydricalcohol is present in an amount within the range of 0.1 to 35 percent byweight of the paper base sheet.

5. An electrostatic writing medium as claimed in claim 4 in which theammonium chloride is present in an amount within the range of 0.1 to 15percent by Weight of the paper Ibase sheet and in which the polyhydricalcohol is present in an amount within the range of 0. 1 to 20 percentby weight of the paper base sheet.

References Cited UNITED STATES PATENTS 2,086,544 7/1937 Dreyfus 117-1395 2,283,558 5/1942 Kline 204-2 2,372,829 4/1945 Holst 204-2 2,651,5859/1953 Lytlc et a1. 117-201 X 2,776,251 1/1957 Schwartz 204-2 2,983,6545/1961 Dalton 204-2 3,011,918 12/1961 Silvernail et a1. 162-138 X3,075,859 1/1963 Ralph et al 117-152 X 3,116,147 12/1963 Uber et a1.162-138 X 3,142,562 7/ 1964 Blake 204-2 X 3,216,853 11/1965 Gess 117-201WILLIAM -D. MARTIN, Primary Examiner. E. J. CABIC, Assistant Examiner.

